Just after initially unveiling Google Play Services, Google has made the APK installable directly from the Play Store.

For those who missed the announcement, Google Play Services is an APK rolling out automatically to devices running Android 2.2+ that will allow Android apps to easily integrate with Google services like Google+. At present, the app (which is in version 1.0 "Asiago") includes components and relevant client APIs for OAuth 2.0 authentication, Google+ sign-in, and Google+ +1 buttons.